1. Field
Embodiments of the invention relate to communication systems and, in particular, to apparatuses, methods, and systems for receiving voice and/or data signals in a mobile or wireless communications network.
2. Description of the Related Art
Mobile communication network nodes, such as user equipment, mobile stations, and base transceiver stations, utilize receivers and/or transceivers to receive, transmit, and/or process data or speech signals.
The Global System for Mobile Communications (GSM) is one of the most common standards for regulating the transfer of data or speech signals between a user equipment and base stations in a mobile communications network. In a GSM network, data is transferred between the user equipment and the base stations as a radio signal over a physical channel which may use frequency and/or time division multiplexing to create a sequence of radio frequency channels and time slots. Each frequency band is divided into time division multiple access frames with, for example, 8 users per frame. Each user is allocated time to send a single burst of information. The user equipment and base station which are in communication may use different frequency bands.
GSM can, in some implementations, use Gaussian minimum shift keying (GMSK) modulation. GMSK is a continuous-phase frequency-shift keying modulation scheme. GMSK modulation uses the phase of the radio signal in order to transmit the data. The phase of the signal is dependent on the frequency of the signal. Such signals generally comprise a train of symbols equally spaced in time. Before the modulation each symbol takes one of two values, conventionally denoted +1 and −1. After GMSK modulation, the signals are comprised of complex valued samples each having an I (in-phase) and a Q (quadrature-phase) component. These signals may be received at an antenna and pre-processed by a front-end section of the receiver. The pre-processing involves amplification and basic filtering. The signal is then sampled at a multiple of the symbol frequency to form a train of complex samples. The samples are then demodulated to form a train of symbol estimates. The symbol estimates are used for further processing and for decoding of the information represented by the symbols.
An alternative phase modulation algorithm is quadrature phase shift keying (QPSK). Phase Shift Keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal. As such, PSK is a form of phase modulation which is accomplished by the use of a discrete number of states. QPSK refers to PSK with four states or phases. With four phases, QPSK can encode two bits per symbol. Examples of QPSK include asynchronous QPSK (AQPSK), offset QPSK (OQPSK), and dual-polarization QPSK (DPQPSK).
Data and voice capacity of communication networks can be enhanced through the optimal selection of modulation techniques for receivers in the network.